This invention relates to lasers and more specifically to thermal lasers (gasdynamic lasers).
Laser is an acronym for light amplification by stimulated emission of radiation. A laser produces a beam in the spectral region broadly defined as optical. The laser beam is coherent electromagnetic radiation having a particular well defined frequency. Ordinary light is incoherent while lasers are coherent. Because of coherence, lasers have extremely small divergence and are highly directional. Also enormous power is generated in a very small wavelength range. This power is focusable on a spot having a diameter of the wavelength itself, and is capable of producing from a 50 kilowatt outburst a radiant power density of 10.sup.12 watts per square centimeter which is about 10.sup.8 times the power density at the surface of the sun. Such power has many uses, such as testing materials, welding, drilling or military applications. Because of the power produced, much research has been directed to the laser field.
Electric discharge, gas dynamic, and chemical lasers are known types of gas lasers. The basic physical process common to them is the competition between stimulated emission and absorption of monochromatic radiation, where the radiation energy corresponds to the difference between two distinct energy levels of an atomic or molecular system. In chemical lasers, the products of highly energetic chemical reactions are formed directly in vibrationally excited states with the upper levels preferentially populated. In gas dynamic lasers, an initially hot gas in thermodynamic equilibrium is rapidly expanded through a supersonic nozzle, and an inversion is formed by differential relaxation processes in the nonequilibrium nozzle flow. In electric discharge lasers, the upper energy level is preferentially populated by collisions with electrons within a gas mixture energized by an electric field.
The laser effect in electric discharge lasers is produced by funneling the gas through an electric field to achieve the desired excited level and produce a laser beam. High energy levels are required to excite the gas to laser producing levels.
Chemical lasers depend on a carefully monitored flow of gases which intersect at precisely the right point at the precise angle with the desired velocity at the right temperature to react to produce the desired laser characteristics. These parameters are only a few of the parameters which must be controlled in order for a chemical laser to function. Controls on each of the parameters are highly complicated in themselves and must be integrated with other complicated controls to produce the laser beam.
Simplest of the three types of lasers to use is the gas dynamic laser. This laser produces the laser beam by means of a rapid gas expansion. This type of laser is simplest because the reactants are generally solid or liquid, and easier to handle and store. However, finding reactants to produce laser action is difficult.
Laser action occurs when two conditions are met: (1) population inversion is achieved and (2) avalanche process of photon amplification is established in a suitable cavity. Population inversion is established in an atomic or molecular system having at least one ground level, and at least two excited levels wherein one of the excited levels has a longer spontaneous emission lifetime than the other excited level. Inversion permits stimulated emission to exceed absorption which results in photon amplification. A more thorough discussion of laser action is found in U.S. Pat. No. 3,543,179 to Wilson incorporated herein by reference.
In spite of the difficulties involved in achieving a laser beam, the power of the laser beam renders the field highly fertile for research. Some of the areas most fertile are those which simplify the generation of a laser beam. The above mentioned electrical discharge lasers, chemical lasers, and gas dynamic lasers are highly complex means of generating the desired laser beam. Efforts in the gas dynamic laser field are made because of the simple operation. Chemical gas generation is a well-known method of simplifying a gas process. The problem now becomes selecting an appropriate fuel or chemical which produces the proper gas for rapid thermal expansion when reacted or burned.
It is possible to pump gas dynamic lasers by use of hydrocarbon/air mixtures. These mixtures are ignited in a combustion chamber and then allowed to expand through a supersonic nozzle so that population inversion occurs. Theoretically, the efficiency of the laser increases with increasing combustion pressure and temperature, and with increasing expansion ratio. The combustion products must contain a high percentage of nitrogen, a minimum percentage of carbon dioxide, and some percentage of water vapor. In addition, the combustion products should not contain any solid particles or highly corrosive gases; however, gases such as carbon monoxide, oxygen and small amounts of hydrogen chloride do not seem to be detrimental to the optical gain. These requirements rule out the use of conventional explosives such as trinitrotoluene, nitrocellulose, and the like, as well as double-base and composite propellants. For military applications, lasers must meet rigid requirements of safety, storage, handling, non-toxicity, etc.
Additionally, for military applications, solid propellants are considered to generate the laser gases mentioned above. The propellant would consist of only the elements carbon, hydrogen, oxygen and nitrogen. However, compounds that can produce high nitrogen, low carbon dioxide and water upon burning are usually unstable, toxic, and hard to store, especially in large quantities. They also have high combustion temperatures that are difficult to use with laser equipment.